In the field of target location and communication technology, it is desirable to accurately transfer signals in the form of radiant energy over an atmospheric path from a transmission source (transmitter) to a remote receiver location (target). The use of atmospheric paths for the transfer of energy increases both the availability and number of transmission routes that are possible for use by a user as opposed to conventional energy transfer systems requiring the use of wires, cables and various transfer mediums other than air.
However, with the use of a new type of energy transfer system come new problems. One limitation upon the efficient transfer of radiant energy over an atmospheric path is the degree to which the transmitted beam can be collimated and accurately aimed at the target. Further limitations of atmospheric energy transfer systems include beam spreading or dispersion, as well as turbulence-induced beam jitter, each of which can compromise the integrity of the energy transfer.
The necessity for positioning accuracy of the transmitter and the receiver in any trans-atmospheric energy transfer system arises from the need for a portion of the transmitted beam to initially be intercepted by the target so as to produce sensible target return, i.e., signal return which exceeds the system threshold and is of sufficient intensity to distinguish the target from extraneous clutter and/or noise events. Suitable target returns will be produced, for example, if the target has highly reflective surfaces generally normal to the transmit beam axis, or incorporates retroreflective or similar materials that reflect light in a direction close to that at which it is incident. The small fraction of transmitter energy which is returned from the target location is directed into the appropriate processing apparatus of the energy transfer system for locating and tracking a specific target. However, it should be noted that the principles of the subject invention may also be employed in communication devices used to accurately transfer information or data from a first location to a second, remote location which can be independent and/or displaceable with respect to the first location.
In the utilization of phase conjugate technology, the small fraction of the transmitted energy which is returned to the transmitter from the target location is directed into a phase conjugate mirror (PCM) assembly. Since the returned signal is generally only a fraction of the strength of the originally transmitted signal, the returned signal is amplified by the PCM assembly. The amplification is typically performed with a modified spatial phase structure in such a manner that the energy signal is returned directly to the target without incurring an appreciable extent of turbulence related loss. The use of PCM techniques with a high gain PCM cell consequently result in the more efficient transfer of energy to the target location as compared to conventional energy transfer systems without the requirement for costly and temperamental precision beam aiming mechanisms.
The general principles of phase conjugate trans-atmospheric propagation of energy to a remote target, as presently known, typically involve the utilization of degenerate four wave mixing in sodium vapor. In these known utilizations, the necessity exists to employ a laser pump source which can be tuned to the 589 nm sodium emission line. To establish the desired tuned emission line in these utilizations, a single longitudinal mode laser pumped dye laser is employed. However, this configuration produces only very low phase conjugate reflectivities. In order to compensate for this problem, a high gain flashlamp pumped dye laser amplifier (G=10.sup.4 /pass) can be inserted in the path between the sodium cell and the target. Although these known experiments did not achieve optimal phase conjugate reflectivity, they nevertheless demonstrated the efficacy of trans-atmospheric phase conjugation up to a distance of approximately 100 meters.